Recently, for improving vehicle fuel efficiency and reducing CO2 emission, the development of technology has been actively made to reduce an engine displacement (engine downsizing) and ensure power performance by a supercharging such as a turbocharger.
Since a turbocharger used in engine downsizing is driven by exhaust energy and has an inertia in a rotating body of a turbo, the turbocharger is characterized in that it is difficult to obtain a supercharging pressure (boost pressure) in a transient operation, as compared with a steady operation.
As illustrated in FIG. 2, even though the displacement is reduced by engine downsizing, an engine torque can be improved by mounting the turbocharger, achieving the same power performance as the large displacement engine. However, in a zone (zone surrounded by a dashed line in FIG. 2) where an engine speed is low, such as an idle operating zone, torque-up cannot be expected because exhaust energy is low and a turbocharger does not work.
Also, as illustrated in FIG. 3, in an engine system (solid line) whose displacement is reduced by engine downsizing, the generation of the engine torque is delayed by supercharging delay (called a turbo lag) of the turbocharger, as compared with a large displacement engine system (dashed line) that generates the same engine torque.
Also, as citation list information related to the invention of this application, there are Patent Documents 1 and 2 and Non-Patent Document 1.